Elasticizing a flexible substrate, such as a sheet of cloth or plastic film, may be accomplished by attaching a tensioned elastic member to it. When the elastic member is allowed to contract, the flexible article wrinkles or shirrs to contract in dimension along with the elastic member. The article can subsequently be stretched as though it were itself elastic. This concept is used, for example, in the manufacture of disposable diapers to provide elastic leg cuffs for snug, leak-resistant, body fit.
There are many commercial processes for combining elastic members with substrate materials. However, reliably combining tensioned elastics with a continuously moving substrate web, as is required in high speed diaper-making systems, has required relatively complex methods and apparatii.
If, for example, elasticity is needed only in machine direction (the direction of travel of a continuously moving web), a continuous elastic ribbon may be fed parallel to the direction of substrate web travel from a source metering the elastic ribbon at a lower rate of speed than the speed of the substrate web. As the elastic ribbon is progressively bonded to the substrate web in the nip of a pair of sealing rolls, the elastic is stretched between its metering source and the sealing rolls. It is stretched an amount depending on the speed differential between the substrate web and the elastic ribbon metering rolls. This represents one of the simplest processes for combining elastics with moving webs.
Where tensioned elastics are applied in cross machine direction or at some other angle to machine direction, more complex systems have been required. For example, the substrate web may be passed through a festoon system whereby the continuously moving web is effectively indexed. That is, the substrate web is stopped for a short time along a portion of its path while the remainder of the web continues to move within a series of accumulation rolls. An elastic ribbon may then be sequentially stretched and bonded in any direction to the temporarily stationary portion of the web.
Where elasticity is desired in curved profiles, such as in elasticized diaper leg cuffs, even more complex processes have been required. For example, an elastic ribbon may be tensioned and then heat deactivated to cause it to become inelastic after it has been elongated. The inelastic ribbon may then be profiled, such as by camming the ribbon in a sinusoidal fashion perpendicular to machine direction as the ribbon is bonded to a substrate web. Because the elastic property is deactivated, the ribbon can be handled without concern for contraction forces disrupting the curved profile before and during bonding. Later, after the shaped elastic has been bonded to the substrate web, a separate heat reactivation step reestablishes the elasticity of the profiled ribbon. Because of the need for heat deactivation and reactivation, elastic material choices are limited. Such processes are complicated by the fact that whenever heat is applied to polymer elastic materials, the temperature must be accurately controlled to avoid interfering with elastic properties. Also, the important properties of spring rate and percent stretch that are available for heat reactivated elastic materials are less than those available for elastics which are not reactivated with heat.
Diaper elastic members are typically either polymer ribbons or laminates of multiple polymer strands combined with nonwoven sheets. Where an elastic member forms a continuous loop with no ends, the elastic member is defined as a garter. Elastic members may generally be bonded to substrates in high speed continuous processes by fusion-sealing, heat-activated adhesive sealing, or pressure sensitive adhesive sealing methods. These methods are more generally classified as thermal bonding and adhesive bonding.
Where a garter is stretched circumferentially and bonded to a substrate to create an elasticized opening, it may be necessary to remove substrate material from inside the garter if a hole was not first created in the substrate before bonding. Commercial processes for removing substrate material inside a garter have generally followed the steps of folding the substrate along a machine direction axis passing through the center of the pattern and then cutting out the substrate material inside the pattern by progressively cutting from the folded edge inward and then back to the folded edge. Although progressive cutting can be done at high speeds by rotary knives, by water jet, or by laser, these are all expensive converting processes. Also hole symmetry about the fold axis is generally required since both sides of a hole are cut simultaneously.
Diaper leg cuffs are often made by abutting two or more elasticized patterns which form less than a full loop. They are joined in order to create a full loop; however, such a composite loop does not have continuous elastics and therefore does not have the capability of distributing the stretch equally around the loop. Only continuous garter elastics have this capability, which is desired for optimum fit. Leg cuffs made by creating a tensioned garter are superior to leg cuffs made by other means of elasticization.
In light of the complexity of known processes for combining tensioned elastic members with moving substrates, and the need for generating diaper leg cuffs made of tensioned garters bonded to a moving substrate, it is an object of the present invention to combine tensioned garters with a continuously moving substrate in a process that simplifies stretching the garter in multiple directions and holding the tensioned garter in a predetermined shape while bonding it to the substrate.
It is a further object of the present invention to bond tensioned garters to a substrate while simultaneously creating a weakened zone at an edge of the garter bond for removal of the material inside the garter, without the need for folding the substrate or introducing external heat to the materials.